These are studies of the pathogenesis and pathology of transplacental and perinatal infections of laboratory animals induced by selected viruses. The virus groups being investigated include parvoviruses, reoviruses, adenoviruses, mumps, and K virus. Several experimental models of virus-induced developmental pathology emerging from these studies include: Granuloprival Cerebellar Hypoplasia (Parvoviruses), Aqueductal Stenosis and Obstructive Hydrocephalus (reovirus 1), encephalitis with selective Neuronitis (reovirus 3, mumps), adrenalitis (mouse adenovirus). These studies reveal, in addition to special viral tropisms, information about the mode of biologic action. Parvoviruses, for example, have a selective affinity for cells with active DNS synthesis attending mitosis. K virus, with its selective attack upon pulmonary vascular endothelium, presents still another example of selective tropism of an animal virus, which is now under exploration. These investigations are aided by the fact that all these viruses studied to date induce pathognomonic inclusions recognizable by light microscopy, immunofluorescence staining, and electron microscopy. Current investigations are focused particularly upon features of transplacental infections, seeking parallels with human disease.